Subsurface reflection imaging as currently practiced in the oil industry attempts to generate an equivalent zero-offset seismic trace by combining the energy observed at many receiver locations. Because of the source and receiver geometries used for marine seismic data acquisition, a seismic trace is never recorded at the zero offset. This is true for marine streamer, Ocean Bottom Cable (OBC) and Ocean Bottom Seismometer (OBS) acquisition geometries. Recording a zero offset or a very nearly zero offset seismic trace allows for more accurate trace interpolation for such processes as AVO (amplitude vs. offset) analysis and SRME (surface reflection multiples elimination), allows the identification of near surface reflections and diffractions and possibly allows multiple generators to be identified. The advantage of recording zero-offset seismic traces has long been recognized. In patent GB2172997, Mathison describes a method that allows the recording of zero-offset data. However, Mathison's method is designed for 2D seismic acquisition.
The vast majority of today's marine seismic acquisition utilizes 3D geometries. FIG. 1 shows a schematic plan view of a conventional 3D marine streamer setup. Vessel 11 tows seismic sources in the form of air gun arrays 12, and also tows receivers in the form of seismic sensor cables 13. FIG. 2 shows a schematic plan view of a conventional 3D OBC acquisition setup. Because of the hardware used to tow multiple seismic streamer cables (FIG. 1), the distance between the center of the source array and center of the nearest receiver group is typically 100 to 150 m. For OBC seismic acquisition, the source arrays are towed over and/or between the seismic cables 21 (FIG. 2). This has the potential to provide zero-offset traces; but shooting a seismic source directly over an OBC sensor can be expected to overdrive the sensor. Because of the potential for overdriving the sensors and because of other geometric advantages, OBC source points are typically located between the OBC receivers. Consequently the nearest source to receiver offsets available with OBC acquisition geometries are on the order of 25 m to 50 m.
Typically marine seismic sources for streamer, OBC and OBS acquisition utilize two air gun arrays, as shown in both FIG. 1 and FIG. 2. The two air gun arrays are shot alternately (i.e. flip-flop fashion) during acquisition of seismic data. Each of the air gun arrays is typically composed of two to four air gun strings 14. Each air gun string typically has five to ten air gun stations. A single air gun or a cluster of air guns is located at each air gun station. Such an air gun array arrangement is schematically shown in FIG. 3. FIG. 3 depicts a single air gun array composed of three air gun strings 31 with seven air gun stations 32 on each air gun string with a near-field sensor 33 located at each air gun station.
As depicted in FIG. 3, common practice is for a sensor to be located at each air gun station. In current practice, the sensor is typically a dynamic pressure sensor which is referred to as a near-field hydrophone. The name near-field hydrophone, NFH, is an obvious simplification since each sensor embedded in an air gun array measures a myriad of signals when any or all of the air guns are fired. The name “near-field hydrophone” has come to be synonymous with any dynamic sensor placed approximately 1 m from the air gun ports that is used to measure pressure and/or particle motion or to measure analogs of pressure and/or particle motion. The term near-field is thus meant, among other things, to exclude the survey sensors 13 and 21.
Near-field sensors are included in air gun arrays as one means of verifying the quality and consistency of the source signature generated by the air gun array (Parkes 1982, Ziolkowski 1997, Brink 1999, Hegna US20080175102). Examples of near-field hydrophone signals from two consecutive shots are shown in FIG. 4. The data in FIG. 4 were generated using the air gun arrangement of FIGS. 1 and 2: two air gun arrays, each array having three strings. The traces in FIG. 4 were gained using a common scalar so the signal amplitudes are relative from trace to trace. It is clear from this display that both the near-field hydrophones associated with the source array being fired (i.e. the active array) and the near-field hydrophones associated with source array that will be fired next (i.e. the inactive array) are dominated by the direct arrivals from the air guns being fired, including the air bubbles for the sensors associated with the active array. The direct arrivals from firing of the port array can be seen on FIG. 4 at 41-44, being respectively direct arrivals at string S6 (41), at string S5 (42); at string S4 (43); and at strings S1, S2 and S3 (44). The air bubble responses can be seen within ovals 45 (bubble at strings S4, S5 and S6 from firing of the port array) and 46 (bubble at strings S1, S2 and S3 from firing of the port array). These direct arrivals obscure the subsurface reflections, hints of which may be seen in the area 47 after time=100 ms for the sensors associated with the inactive array. (The horizontal axis scale is time in ms.)
One particularly effective means of using the near-field sensors to quality control air gun arrays is on a shot by shot basis to vertically sum (i.e. vertical sum and vertical stack are synonymous signal processing techniques where common time samples from two or more times series are summed or averaged together to create an output time series) the signals from the near-field sensors associated with the active array and then to display these summed signals in a density style display with the signals sorted into port and starboard order. This type of analysis is shown in FIG. 5. The variations between the port and starboard arrays are associated with variations in the individual near-field hydrophones and the condition of their associated analog signal paths. For this type of display, any significant variations in the array geometry or variations in individual air gun output will result in easily identified discontinuities in the display.
A comparable type of analysis can be done using the near-field hydrophones associated with the inactive array (FIG. 6). This type of display is typically not generated because after the arrival of the direct arrivals from the active array, the display is contaminated with what appear to be signals associated with the water bottom and the subsurface geology. The contamination of air gun source signatures by the water bottom reflection in shallow waters is well known for near-field hydrophones (Ziolkowski 1997 and Kragh 2000) and for mini-streamers (Hargreaves 1984 and Amundsen 2000).